Metal flake



US. Cl. 106290 6 Claims ABSTRACT OF THE DISCLOSURE Finely divided metalparticles, such as zinc dust particles, are flattened by mechanicalflattening forces in an aqueous media to provide metal flakes. Theaqueous media includes a polishing agent which is capable of exhaustingitself in a thin -film over the surface of the metal particles, ametallurgical flux which is capable of dissolving metal oxides and saltswithout being strongly reactive with the base metal, and a Weldmentinhibitor selected from the group consisting of strong nitrogenous basicferrous metal corrosion inhibitors, aliphatic sulfur compounds, aromaticsulfur compounds, aromatic aldehydes, and aromatic ketones. 'Ihe flakesare eminently suitable in decorative and protective paints.

The present invention relates to flattened particles of metal and tomethods and materials used in the fabrication thereof. Moreparticularly, the invention relates to fabrication of small metal flakesuseful in protective and decorative paints.

It is known in the art to make metal flakes, such as zinc flakes, bygrinding and polishing in a ball mill or other similar apparatus capableof imparting mechanical energy to metal particles admixed with an impactmaterial such as iron shot. This operation, which is described, forexample, in US. Patent No. 1,954,462 and No. 2,080,346, is carried outin a non-aqueous medium, which consists of a solvent such as ahydrocarbon solvent, chlorinated hydrocarbon, alcohol, ketone or thelike, and which dissolves water insoluble fatty lubricants such as oils,fatty acids and the like, which are necessary for the polishing andflattening of the flakes.

It is also known in the art to encase the zinc particles resulting fromthe above described procedures with micacious materials to protect theflakes as described, for example, in my US. Patent No. 2,528,034.

The old process described above, using volatile or organic solvents, issubject to several major disadvantages. The flammable hydrocarbonsolvents such as naptha constitute a serious and dangerous fire hazardwhich is objected to in commercial plants. The use of non-flammable,chlorinated hydrocarbons is subject to other disadvantages. Thesematerials are highly toxic; the vapors are dangerous; they are expensiveand heavy, which results in a high cost per unit volume. Because of thehigh cost, these organic liquids must be recovered by distillation.Recovery by distillation is not a simple matter because there is areaction between the finely divided zinc and the plating agents whichare dissolved in the organic solvent which raise the boiling point andwhich retire some portion of the solvent used. This, together withunavoidable handling losses and drag-out, etc., leads to a relativelyhigh cost for solvent use and recovery.

Other disadvantages associated with the prior art process are due to thefact that it is diflicult to make a sufficiently fine flake forcommercial use. In the process of polishing and flattening the flakes,chemically clean and active surfaces are created which tend to weld ormate with the clean, reactive surfaces of other flakes with the resultthat larger flakes grow by accretion from smaller United States PatentSee flakes and these weldments are such that the larger flakes becomesolid entities which can no longer be broken down into their componentparts. The end result of this process has been to make it difficult orimpossible to produce a really fine flake suitable for use in paintsrich in the metal flake. The enlarged flakes of the earlier processesare extremely heavy and, because of their larger size, tend to sink inthe paint vehicle. This problem is particularly troublesome with metalswhich have a tendency to weld, such as zinc, tin, cadmium and lead butis less pronounced with the noble metals, such as gold and silver, andwith metal which behave similarly such as copper, brass and stainlesssteels.

Another disadvantage of the earlier flakes lies in the fact that whenmade up into paint, the clean surfaces of the flakes tend to bunchtogether so that the flakes tend to smear out in that paint film likecards in a sticky deck. This sticky adherence of flake to flake producesa poor brushing-out paint. Attempts to overcome this by encasing,enclosing or encapsulating the flakes with mica are successful, but suchmica-coated flakes have a low coeflicient of friction which leads to theformation of brush marks in the painted film.

Not only do the highly polished bright flakes tend to stick to oneanother and grow by accretion, but they also will adhere to other cleanmetal surfaces such as the surfaces of the shot used as impact medium.The more potent and effective the polishing agents used previously, themore effective and more important the effect of plating finely dividedflakes to the impacting material. In weekin, week-out operation, saidshot will collect appreciable accretions of metal which lowers metalutilization and limits the amount of polishing which the flake can takebecause the coated impact shot which becomes rougher and rougher as thesurface grows does not polish as readily as the smooth, highly polisheduncoated shot. The process tends to operate in a vicious circle in that,as the shot gets larger and the surface gets rougher, the process ofaccretion speeds up and the shot grows ever larger, robbing the chargeof free or useable metal. The rougher shot must be tumbled longer andlonger with the metal to achieve the proper polish, and the longer thecycle, the less the useable metal remaining in the charge when the millis emptied. This means that it is absolutely necessary that the steelshot or other impacting medium be cleaned in acid or otherwise uncoatedperiodically. This is particularly true in the case of zinc metal.

The use of solvents, heavy iron shot as polishing and flattening medium,and long polishing cycles may result in the formation of colloidal orultra-fine metal particles which cannot be polished. In the case ofzinc, this colloidally-fine material, called black stuff, is wasted fromthe useable metal and lost to the operation.

Despite the disadvantage of the prior art set out above, zinc flake asmade by this older method had a good deal of commercial merit. This wasproven by the painting of large structures such as bridges across theMississippi, large 80,000 'barrel gasoline tanks, hot input lines andthe like with zinc flakes made into paints. In these applications thezinc flake paint was found to have good properties in protection againstcorrosion. In the majority of these earlier applications the coating wasapplied to roughened, sand blasted steel where the substrate had aninnate and very considerable roughness. For other applications, such asthe coating of cold rolled steel plate such as is used in the automotiveindustries, the extremely smooth and highly polished steel surfaces areevidently without essential tooth. It has been found that the relativelycoarse zinc flake paints made by earlier methods did not brush outsatisfactorily on such smooth, polished surfaces.

It is an object of the invention to provide improved methods andmaterials for fabricating metal flakes. It is a further object of thepresent invention to provide methods and materials for making metalflakes which have improved properties and to provide such flakesthemselves. It is still a further object of the invention to providepaint compositions rich in metal flake and having improved properties.It is yet a further object of the invention to provide methods andmaterials for making metal flakes which are smaller than thoseheretofore attained. It is still a further object of the invention toprovide improved and superior coating materials for protecting metalflakes to better insulate same against an aqueous environment and toprovide paints made therefrom with better brushability.

These and other objects of the invention which will become apparent inview of the following detailed description thereof are achieved byutilizing an aqueous media which includes a flux and a polishing agentin a method of flaking or flattening of metal particles by thesubjection thereof to mechanical forces which flatten the particles intoflake form without causing substantial adhesion thereof. The presentinvention relates only to aqueous systems and it is therefore consideredexpedient to discuss several of the known problems and solution thereofin this general type of process.

If any very finely divided powder of a reactive metal, such as zinc orthe like, is mixed with water, a reaction occurs "between the metalpowder and the water leading to the formation of metal hydroxide andhydrogen gas. If a reactive metal is tumbled in water in a closed steelbarrel for lengthy periods, highly explosive pressures may be generatedwhich are capable of blowing up the steel mill. The formation of metalhydroxide such as zinc hydroxide occurs as a fine white powder, and thiscombined zinc is lost to the operation. Because the polishing of largequantities of metal powder such as zinc requires fairly lengthyprocessing times, the formation of this zinc hydroxide, which proceedswith the passage of time, can result in a serious loss to make theprocess uneconomical assuming the question of the gas generation can beovercome and adequate polishing can be made to occur. To summarize,therefore, the use of water in the manufacture of bright flake hashithertofore been regarded as diflicult or impossible because it has notbeen possible to control the formation of hydrogen gas nor has it beenpossible to polish the very fine metal powder in a water medium, nor hasit been possible to prevent the formation of metal hydroxides and otherdegradation products of extremely fine dust used.

Some of these problems associated iwth the admixture of extremely finepowder in water are overcome in an entirely different process, namely aprocess for mechanically plating fine metal powders onto steel objectsin an aqueous medium using certain materials which act to film the metalparticles and shield them from the aqueous environment. However, sincethis method is primaril a method for applying metal coatings to objects,the impact media is heavily plated with metal and even before one or tworuns are completed, the impactor becomes unuseable and grows to manytimes its original size, tying up large quantities of metal as ametallic coating on the impactors. In such mechanical platingoperations, a very small amount of metal powder is used and the timecycles are quite short, in the order of one hour. On the other hand,manufacturing bright flake, large quantities of metal are used and timecycles range up to 48 hours and more. If any attempt is made, therefore,to use mechanical plating techniques with these large concentrations ofmetal, the tendency to coat on the iron shot becomes enormouslyincreased and the problem is greatly aggravated to the extent that theprocess is not workable. This problem existed, although in a lesserdegree, in the buildup of metallic coatings by mechanical platingmethods and led to the substitution of the metallic impact medium byglass beads, or other non-metallic surfaces which do not accept a metalcoating. It has been found, however, that glass beads cannot be used forthe manufacture of flake because of the large volume of zinc powderused. The glass beads do not have suflicient weight and energy tooperate effectively in a high density water slurry of metal powder.

Furthermore, where aqueous systems are employed in the buildup ofmetallic coatings, the gas generation problem can readily be controlledbecause of the small volume of metal powder used, the rapidity withwhich it is converted into a metallic coating on the objects, and theshort overall time cycle. In the manufacture of metallic flake pigments,where the polishing cycles are perhaps 50 times as long or longer, thegas generation becomes a very major problem. For these and otherreasons, it will be seen that the methods used in mechanical plating arenot applicable to the manufacture of bright metal flake.

It has been discovered that by utilizing a flux and a polishing agentwhich films on the metal in an aqueous medium, bright metal flakes invery small sizes are obtainable without the evolution of gas and withoutweldment of the small metal flakes produced. This aqueous environmentcreates conditions for covering the metal particles with a film suchthat each is insulated from the water with which it would otherwisereact, for removing oxides or metal degradation products from theirsurface, and for preventing weldment of one to another, to the mill, orto the impact media or any other surface with which they may come incontact.

The flux may be any of the conventional fluxes used in metallurgicalprocesses but the less corrosive fluxes are preferred. The amount offlux is not apparently critical and is primarily a matter of economy anddependent on the dirtiness of the metal used. In general, however, aminimum of /2 to 1% by weight, based on the weight of the metal powderto be flattened, may be used although amounts well above 1% are useable.The flux generally is capable of dissolving metal oxides or saltswithout being strongly reactive with the base metal. For any given metala host of such fluxes is known and each is believed operable herein.Organic acids, such as citric, tartaric, hydroxyacetic and the like areexcellent as are inorganic acids such as phosphoric. The alkali salts ofthese acids such as zinc chloride and ammonium chloride are alsosuitable. In addition, ammonium hydroxide or sodium or potassiumhydroxide in weak solution are suitable. In general then, the use offluxes known to function as same for the metal in question are suitable.In view of the fact that the metal is finely divided, somewhat morediluted use than normal is preferred to prevent excessive dissolution ofthe base metal.

The polishing agent is a film forming material or oil which is capableof exhausting itself in a thin film on the metal particle. The filmformer may be water soluble or insoluble and may be used in conjunctionwith a detergent which aids in wetting the metal with the film former inthe aqueous medium. In fact, several detergents are film formers and maybe used by themselves in this capacity.

While suitable oils fall into various chemical classes, it is thephysical property of the oil to exhaust itself in a thin film on themetal that is controlling. The only chemical criterion, of course, isthat the film former does not destroy the metal. I have disclosedsuitable film forming materials in my earlier Patents Nos. 2,698,808;2,640,002; Re 23,861; 3,132,043; and 3,023,127. There are virtuallyhundreds of materials such as those disclosed therein which have thephysical property of filming a clean metal surface and interposing aphysical barrier between the metal surface and its environment. Theability to film the metal is apparently enhanced in all cases Where themetal is clean and the flux aids in this regard. There is, of course,some competition between the flux and film former but this is minimizedbecause when the metal surface is not clean, the affinity between thefilmer and the metal is at a minimum and the effectiveness of the fluxcleaner is maximum. As soon as the metal surface is clean, the filmformers aflinity therefore is increased and the film formed serves notonly to insulate the clean metal from the aqueous media, but fromunwanted further action by the polishing agent. The amount of polishingagent necessary to film the metal will, of course, vary on the amountand size of the metal. In general, however, a minimum amount of fromabout /2 to 1% by weight of the metal is believed to be necessary forcoating fine dusts having a high surface/ volume relationship. Anyamount of film former substantially above the amount necessary to filmthe metal, is operable and the maximum is largely a matter of economy.In the usual case, therefore, a generous excess is used over thetheoretical minimumwhich would be the amount necessary to form amonomolecular film over the entire surface area of the product to ensurethat a suitable protective film is formed.

Various oily materials, such as mineral oils, oily hydrocarbons, oilyorganic acids, amines and amides, quaternary ammonium compounds,silicone oils and others may be used. The organic acids are particularlyattractive in that, in addition to satisfying the requirements of thefilm former, they may also qualify as a flux, thus obviating the needfor the use of a separate flux material. Long chain fatty acids, bothsaturated and unsaturated, are particularly suitable in this respect aswell as several various derivatives thereof. Fatty acids having at leastsix carbon atoms are suitable and of these, isostearic acid isexcellent. The alkyl esters of these acids, particularly the lower alkylesters thereof, such as butyl oleate and butyl stearate are suitable.Additionally, fatty acid esters of lower acids, such as butyl acetate,are useful. Similarly, fatty amines and amides of C and higher acids areeffective. The organic oils, such as linseed oil, tung oil, palm oil,safllower oil, corn oil, etc., which are glyceryl esters of the higherfatty acids are also suitable. In addition, oily quaternary ammoniumcompounds, such as alkyl methyl ammonium chloride, are suitable as areoily aromatic materials such as safrole. Still another useful class ofmaterials includes oily synthetic resins such as polyvinyl acetate in afinely dispersed state. In short, the lubricants function is apparentlyentirely physical and any material capable of exhausting itself on thesurface of the metal, either with or without the aid of a surfactant, isapparently suitable. In addition to this physical function, the oil mayact as a flux and it is to be understood that, as used herein the termflux includes a film former which acts additionally as a flux.

Where the metals to be flattened are prone to adhere to one another, aweldment inhibitor is used in the aqueous medium to prevent adhesion.The inhibitor which prevents the adhesion or weldment of flakes to oneanother or to the impact media or other surfaces may be any of severalmaterials which are capable of exhausting themselves into a tough filmon the metal. It is to be noted that whereas base metals having lowrecrystallization temperatures like zinc, cadmium, lead, and tin have avery pronounced tendency to cold weld, with noble metals such as goldand silver, and with metals which behave nobly such as titanium andstainless steel, the tendency is much less pronounced. However, theinhibitors do reduce the tendency of the flakes of all of the metals tocold weld.

There are various materials which form protective conversion coatings byreaction with the surface of the metal. Hexavalent chromium, forexample, functions in this manner to produce a zinc chromate finish onzinc particles. Chrome salts, however, attack zin and other metals andare thus not suitable as inhibitors in view of the fine size of themetal flakes. Antimony trichloride and other materials which similarlyattack the metal to form a conversion coating are therefore not suitedas inhibitors in the processing of fine particles.

Inhibitors which are suitable are the various materials which aresubstantially inert to the base metal and are capable of forming toughtenacious films on the metal which is not partable under the processconditions. A number of suitable materials are known which possess suchproperties. A class of inhibitor which has been found to be suitable andwhich does not attack even the reactive metals such as zinc, consists ofan organic material having a hydrocarbon moiety and a polar or ionizablemoiety. Any of the strong nitrogenous organic basic inhibitors disclosedin U.S. Patent No. 2,217,921 are considered applicable and particularlywhen used in conjunction with a water soluble high molecular weightpolyethylene glycol having a molecular weight of at least 15,000, asdisclosed in U.S. Patent No. 3,141,780.

Other inhibitors which are suitable include aliphatic and aromaticsulfur compounds such as thiourea, o-tolylthiourea, p-tolylthiourea,phenyl thiourea, p-thiocresol, thiophenol and aromatic aldehydes andketones such as rn-toluylaldehyde and butyrophenone.

The inhibitors are relatively powerful and as little as A% or even lessby weight of the metal powder may be suitable. In some cases, as forexample in the case of a noble metal, the inhibitor can be omittedaltogether and the film former relied upon to prevent weldment. Wherethe metal tends to weld, however, some inhibitor is required. While alittle of the inhibitor goes a long way, excess amounts seem to do noharm and, accordingly, amounts substantially beyond the bare minimumnecessary are conveniently employed to ensure that the end result isobtained.

If a reactive metal flake is placed in hostile environment which willattack it, the flake must be protected by application of a tenacious andcontinuous film which will serve as a barrier between the flake and theenvironment. These films fall into two classifications: partable filmsand non-partable films. Whether a film is partable or not will depend tosome extent on the disruptive forces available for flattening andpolishing the flake. Such films are essential for the production ofbrightly polished metal flakes having smooth surfaces. If the flakes cancold-weld together and the film is non-partable or creates a surfacehostile to the cold-welding process, cold-welding does not take place.It is a function of the inhibitor to prevent such cold-welding and toprevent the cold-welding of particle to particle or particle to impactmaterial. In the case of those metal powders which do not exhibit thetendency to plate or cold-Weld together, the requirement for theinhibitor is minimized but the requirement for the partable film remainsas this lubricates and assists in the polishing and flaking operation.

In order to make flattened and polished metal flakes, the metal powderis subjected to mechanical action in the presence of the aqueous mediawhich contains the oil and flux and optional weldment inhibitor. This isconveniently achieved by subjecting the particles to the mechanicalaction of impacting media, such as metal shot, in a ball mill of thetype mentioned in my aforementioned patents.

The metals which can be polished and flattened according to theinvention include zinc and metals below zinc in the electromotive seriessuch as brass, bronze, aluminum bronze, etc. and exclude aluminum andother metals above zinc in the series. The metals which can be flattenedaccording to the invention fall into two classesthose which tend tostick or weld together and those which do not. The former class includesmetals having low recrystallization temperatures such as zinc, tin,cadmium and lead which have recrystallization temperatures of about roomtemperature or below or from about 50 F. to F. The latter class includesmetals having a high recrystallization temperature such as stainlesssteel, copper, copper alloys such as brass, bronze and aluminum bronzewhich have recrystallization temperatures above room or over about 100F.

The principal use for the flattened and polished particles according tothe invention is in decorative and protective paints. Accordingly, thefinal size and size distribution of the materials is usually significantand these factors, in turn, are largely dependent on the size of thestarting material. With metals which do not tend to stick, the size ofthe starting material is not too important because a grinding aid can beemployed during the process so that larger pieces formed by flatteningare reduced to a size suitable for paints. With sticky materials,however, it is preferable to start with fine materials because evenunder the best of circumstances there is some weldment and the use of asmaller size starting material will result in the recovery of a largeryield of particles of a suitable small size.

The impact media should be dense in order to flatten the dust into flakeform in a relatively short time. For this reason, metal impactors arepreferred. Suitable impactors are spheres such as iron shot and thelike. The spheres are massive with respect to the dust and may beconveniently A3" or less in diameter and larger. Commercial scale runshave been effected with iron shot of from A" to /1" diameter.

The amount of impact media can be varied widely and this variation canbe used to effectuate certain results with respect to the configurationof the end product.

For example, if one hundred pounds of Zinc dust, together with all thenecessary chemicals for the process, were tumbled with one hundredpounds of iron shot of approximately Ms diameter in a tumbling barrel,the iron shot would be more or less imbedded in zinc dust, and itsfreedom of movement would be retarded by the mass of this dust, whichwould act to cushion the blows required for flaking. The flakingoperation would proceed slowly and the flakes would tend to be onlylightly flattened, resulting in a more cubical type of flake. To reducethe whole hundred pounds of zinc dust to a bright, polished cubical typeof flake might require, let us say, 6 days or 144 hours, assuming thiswere done in a small barrel of say 6" diameter. If the operation weredone in a barrel of say 4 feet diameter, the time would be reducedbecause of the greater pressures, the higher temperatures generated, andthe longer length of the slide or active zone of the barrel.

If, however, instead of one hundred pounds of zinc dust, one pound ofzinc dust was selected and used with the same one hundred pounds of ironshot, all the other conditions remaining exactly the same, the flakingand polishing operation would proceed very much faster so that a veryhighly polished, flat flake might be produced in say 1.44 hours oftumbling time. At the end of 144 hours of tumbling time, the productionwould be (neglecting losses and downtime of the barrel) one hundredpounds of finished flake in each instance.

There would be a substantial difference in the characteristics of theflake produced by these two procedures. The one pound charges wouldresult in a very flat, very highly polished, but very coarse flake. Thiscoarseness or enlargement of the flake would be the result of coldwelding of one flake on to another and continuing the process untildozens or hundreds of flakes would be cold welded and cohered into asingle flake, which would constitute a united mass of metal which couldunder no circumstances be reduced to its original components. In theother case, there would be only slight cold welding due to the reducedseverity and frequency of impact, and the flake would be much closer tothe original fineness of the zinc dust. The flakes would be lessflattened and less bright, and there would be a greater diversity ofparticle size which is highly desirable in a paint pigment. In the caseof the one pound of zinc loading, the internal friction of the revolvingcharge will be quite low, and the round steel balls will roll on oneanother almost as if no zinc were present. In a major percentage of theimpacts there may be only a single flake between two rubbing orcolliding steel balls. Under these circumstances, the probability of theflake cold welding to the surface of one or the other of the steel 8balls is inordinately high. Once cold welded to the ball it becomes apart of it and cannot be removed, so that the flake is lost to theoperation.

Exactly the same mechanism may explain the enlargement of individualflakes. If the flake does not cold weld to the impacting ball, it willbe flattened and rolled out into a very thin flake of maximum surfacearea. If two or more particles are caught in an impact they may be coldwelded one to another to produce a larger flake or alternatively, aflake previously flattened may at a later impact be caught with a round,unflattened particle of zinc dust which will be rolled out and coldwelded onto the surface of the previously flattened flake.

The inhibitor is added to the charge to prevent cold welding. Obviously,much greater demands are made on the inhibitor in the case of the onepound zinc loading than would be the case in the one hundred pound zincloading. Therefore, as a practical matter, the quantity of inhibitor inthe case of the low zinc loading would have to be greatly increased ifexorbitant losses are to be prevented. Simultaneously with the increasein inhibitor, the flux would be reduced because this reagent promotescold welding. The amount of filming agent or lubricant which helps topromote luster and polish, could be reduced because there would be lessnecessity for it in the case of the low loading of zinc. At the sametime, grinding agent could advantageously be increased to promote thebreakdown (i.e. fragmentation) of enlarged particles.

The above description will serve to explain the interrelationship ofmetal particle, impact media, load and the formulation of the flakingcharge. Quite obviously, the highly enhanced requirement for labor, andexpensive chemicals, like inhibitor, to say nothing of the downtime ofthe barrel after each pound loading, would be entirely impractical froma commercial point of view, but not necessarily from a technical pointof view.

It is important to note that all of the above remarks apply to thosemetal powders that will cold weld together under the mill conditionsprevailing. For metal powders that do not weld together, then thereverse is true and low loading of metal powder to impact medium willresult in the production of ultrafine flakes of very high quality andpolish. For this reason, entirely different standards are applicable,and ratios of say brass powder to impact material that would be entirelysatisfactory for brass paint pigment, would be entirely unsatisfactoryfor zinc pigment formulation.

In any event, it can generally be said that for making paint-sizeparticles, that is, a product which is at least minus one hundred meshand at least 50% minus 200 mesh, the amount of impact media, preferablyin the form of at least substantially spherical shot based on the amountof dust to be flattened, may be as follows:

For metals that tend to stick or weld, a weight ratio of from 10/ l to2/1 while a ratio of from 20/1 to 0.5/1 is suitable. For metals which donot tend to weld, ratios of from 20 to 1 to 2.5/1 are preferred andratios of from 35/1 to 1/1 are suitable.

Other materials which may be added in generally minor proportions, andall of which are optional, include defoamers, buffering compounds tomaintain acid pH during the process; grinding aids to reduce the size offlakes produced; and detergents which are usefuf in distributing thefilm former on the metal particle.

The grinding agent is not essential in the environment but it isgenerally helpful when present in minor amounts as it facilitatesreduction of the particle size and aids in producing a finer pigment. Tobe very effective in paints, the flake size should not exceed 325 meshand should, preferably, be much finer. Having regard to the geometry ofthe flake, which is practically all surface, even an extremely smallround particle can be converted into a flake which has substantiallymore surface area and consequently will not pass through a screen whichthe undistorted zinc particle can go through readily. This means that agrinding agent which facilitates the breakdown of flake and leads to amixture of flake sizes aids in producing a paint of good brushability.There are a considerable number of grinding agents on the market whichare used in the grinding of paint pigments and which are suitable foruse in this invention. Examples of preferred form grinding agents aresilicon carbide, cutting sand, alumina, and the like.

The type and character of the detergent or surface active agent is notparticularly critical. A wetting surface active agent such as Tergitolfrom Union Carbide is suitable. Cationic and non-ionic surfactants arepreferred. It is preferred to operate at an acidic pH.

The invention is further described in the examples which follow whichconstitute preferred embodiments thereof:

The buffering compound contains one part by weight of amine to 2% partsby weight of the acid in the mix. Tenlo 70 is an oil soluble non ionicsurfactant. To approximately 1600 grams of the aqueous media were addedabout 10 pounds of /g" O.D. zinc-coated iron shot. The zinc coating isfrom previous runs. The uncoated shot is available commercially for shotblasting as No. 1110. About 3 pounds of fine zinc dust (having anaverage particle size of about 1-3 microns) was added to the media andthe total added to a hexagonal rubber lined barrel 6 across the slatsand approximately 8" high, and having a capacity of about quarts. Themill was closed and run at 67 r.p.m. for 48 hours. On opening thebarrel, the zinc powder had become converted to a very fine, highlypolished zinc flake. There was no gas pressure on opening the barrel.The zinc flake was washed in water by several decantations, the flakebeing settled between each decantation. The total zinc charged was 1590grams. The total minus 200 mesh material recovered was 1533 grams;1115200 mesh flakes weight 19 grams. Total useable flake recovered was97.8% There Was no gas pressure on opening the barrel. There was nobuild up of zinc on the iron shot, or on the sides of the barrel. Therewas little or no cold welding of one flake to another.

-It is believed that the toluidine base corrosion inhibitor and filmingagent provides tough non-fracturing films which together with theisostearic acid, combine in shielding the metal flake from the waterenvironment, and thereby prevent the plating of one clean metal flake onanother and the plating of the flake on the impact medium. The totalrecovery of 98.7% would be greatly reduced by deposition of zinc on thesteel impact media were it not for the tough protective films.

Example H A final flake with still better total recovery was made byadding one pound of minus 24 mesh of silicon carbide to Example I. Theeffect of the silicon carbide, which is very abrasive, increased thepercentage of minus 325 mesh in the minus 200 mesh material, but therewas a slight reduction in the luster of the resulting flake.

Example III Example I was repeated using three pounds of very fine zincpowder, but with the addition of 10 grams of finely ground jasperinemica. This mica was stamped onto the flake. On opening the barrel a verybeautiful flake was obtained which decanted and filtered very fast.There was no gas on opening the barrel and the recovery was excellent.

Example IV Example III was repeated, but with 25 grams of mica added.The product did not settle quite as well as previous runs, and wasslower filtering. The recovered flake, however, was excellent. There wasno gassing on opening the barrel.

Example V Example I was repeated, but with the addition of 25 grams oflead stearate as a film former. The purpose of this was to further thefilm action on the flakes to further protect them from the aqueousenvironment and to produce a final flake of top quality. All otherconditions remain the same as in Example I. There was no gas on openingthe barrel. The flake recovered was very pretty, settled very well,filtered fast, and was fine in particle size. The recovery wasexcellent.

about 4500 grams of No. 1110 impacting media. About 1400 grams of finecopper powder was added to the media and treated for 48 hours as inExample I.

There was no gassing on opening the barrel. All of the flakes wereextremely fine, too fine for ordinary mesh analysis. Recovery appearedto be virtually The material was easily washed and filtered fast, and nodifficulty was experienced in drying.

Example VII Three pounds of minus 200 mesh 70/30 brass was substitutedfor the copper powder in the copper flaking example above. All otherconditions remain the same. The resulting brass powder was very bright,very fine and filtered readily.

Example VIH Type 316 stainless steel powder minus 325 mesh, obtainedfrom Vanadium-Alloy Steel of Latrobe,

Pa. pounds 1 No. 1110 iron shot do 10 Tenlo 70 grinding compound cc 30Isostearic acid 30 Polyoxyethylene inhibitor Buifering compound cc 100Silicone defoamer drops 5 Finely ground jasperine mica "grams 25 Theflaking and polishing operation lasted 48 hours. There was no gassing onopening. The stainless steel flake was pretty and well flaked. It willbe noted that no corrosion inhibitor compound was used in this testbecause the stainless steel has no tendency for the flakes to plate onthemselves or on the impact material.

As has been mentioned heretofore, particles produced according to theinvention are particularly useful in decorative and protective paints.For this use the particles are quite small and are generally at leastabout 75% minus 100 mesh and at least about 50% minus 200 mesh.Particles of metals with low recrystallization temperatures of from 50to 100 F. in this size range, are considered to be new and to constituteone embodiment of the present invention. The following examplesdemonstrate the use of the invention to make such metal-rich paints.

Example IX In this and the following examples a rubber lined octagonshaped tumbling barrel 3.0 inches in diameter Pounds Water 412 Tenlo 70surfactant 4.27

Citric acid-polyoxyethylene oxide amide buffering flux 10.0 Siliconeantifoamer 0.8 Toluidine based inhibitor 9.4

Mesh: Percent 200 99.3 250 97.8 -325 72.1 -400 60.3

Example X The following charge was added to the tumbling barrel:

Pounds Tenlo 70 8.61

Buffering flux of citric acid, and polyoxyethylene oxide amide 14.98Silicon antifoamer 1.61 Toluidine based inhibitor 9.45 Water 346.00Jasperine mica 20.72 No. 1110 iron shot 1500.00 Minus 325 mesh AA zincdust 700.00

The charge was tumbled for 25 hours. The flake produced was bright andhad the following analysis:

Mesh: Percent l 100 200 80 -325 40 Example XI The formulation of ExampleIX was followed except Mesh: Percent 200 99.4 -250 98.8 325 68.5 --40045.9

Although the above examples specify the use of a toluidine-basedweldment inhibitor, other weldment inhibitors such as those mentionedherein can be similarly employed to form tough, non-partable films onthe metal flakes.

The use of metal particles produced according to the invention in metalrich paints has been demonstrated with the zinc flakes produced byExamples IV and XI. The flakes were washed and dried and formulated intopaint. The vehicle is the ester reaction product of Araldite 6084 (Ciba)and dehydrated Castor Oil Fatty Acids, all as fully described on page 2of Ciba Technical Bulletin Epoxy Resins, Araldite 6084. This vehicle wasfound to give best results with standard zinc dust paints and was thusselected for comparison purposes. Five formulations of the zinc flakesof Examples IX adn XI were made up such that the dried paint filmcontained 72%, 84%, 88% and 92% of zinc metal. The paint was applied inone coat on mild carbon steel panels. One half of the painted steelpanel was overcoated with an epoxypolyamide overcoat paint meeting therequirements of military specification MIL-C- 22750A (Wep). Both halvesof the panel were diagonally scored with a blunt wide scribe making agroove through the zinc coating to the underlying steel base. Thesepanels were then exposed in duplicate in a standard salt fog cabinetwhere the performance was compared with coatings of a commerciallyavailable zinc-rich epoxy paint and similar formulations made of highgrade fine zinc dust that was found to give excellent results inprevious tests in salt spray. The comparative tests were continued for1000 hours during which the panels were examined at regular timeintervals. Standard tests for zinc rich paints for the automotiveindustry are continued for only 500 hours. The test results aretabulated below. In the table, 10 is a perfect score indicating rustfree. A score of 0 indicates 100% rust. In this regard it should benoted that once the score is zero, further rusting proceeds at such arate as to not be analogous or comparable to the rusting rate from score10 to score 0. For this reason, several panels scoring 0 were not testedfurther.

FLAKES RUSTING AT VARIOUS TIME INTERVALS 600 hours 750 hours 1000 hoursOne coat Over coat One coat Over coat One coat Over coat One coat Overcoat One coat Over coat hours 410 hours Amount of zinc in paint film,percent Example IX:

8 10 5 10 l0 l0 l0 10 10 10 10 10 l0 10 10 0 0 0 0 0 0 5 5 0 10 9 9 921O 1O 5 Commercial, 92% 10 1O 0 H r-n- H omenoo acumen Someone that .10pounds of finely ground white asperine mica was added and the amount ofinhibitor was increased to 10.88- pounds. The charge was tumbled for 15hours and produced bright flat flakes of lustrous polished appearancehaving the following Tyler screen analysis:

The tabulated information shows clearly that paint made with zinc flakesaccording to the present invention is far more corrosion resistant thansimilarly loaded commercial paint or paint similarly loaded with zincdust. It also shows that paint made with less zinc flakes is as good asor better than other paints more heavily loaded with zinc.

There is a class of zinc pigments in which the vehicle containschemicals such as silicates which attack the zinc and the resultantdecomposition products form a cement which serves to bind the Zinc tothe base. These products are all unattractive in appearance as are allzinc dust paints. The highly polished flakes of this invention do notrequire reaction with the vehicle to secure the necessary adherence tothe substrate and integrity of the coating film.

Zinc flakes according to the invention have, of course, many uses otherthan in paint and the particular configuration of flakes according tothe invention renders these flakes particularly advantageous and evenunique Among the particularly advantageous and even unique uses of theseflakes are: pipe threading greases; clean, film-free flakes for use inprecipitation reactions such as the purification of zinc sulfatesolutions in zinc electrowinning; and the provision of sherardized zinccoating rendered facile by the ease with which these flakes can besherardized by heat.

What is claimed is:

1. A process of flattening and polishing finely divided metal particlescomprising: providing finely divided metal particles of a metal nothigher than zinc in the electromotive series in an aqueous mediacomprising water, at least /z% by weight, based on the weight of saidmetal particles, of a polishing agent comprising an oily materialcapable of exhausting itself in a thin film on the surface of the metalparticles, at least /2% by weight, based on the weight of said metalparticles, of a flux to remove metal oxides from the surface of themetal particles, and a welclment inhibitor present in an amountsuflicient to prevent weldment of flakes to one another and selectedfrom the group consisting of the reaction product of formaldehyde ando-toluidine, thiourea, Q-

tolylthiourea, p-tolylthiourea, phenyl thiourea, p-thiocresol,thiophenol, m-toluylaldehyde and butyrophenone; and subjecting theparticles to mechanical flattening forces in the presence of saidaqueous media to form polished flakes of said particles wtihout Weldmentof said flakes to one another.

2. A process according to claim 1 wherein said flux and said polishingagent are each present in an amount of at least 1% by weight based onthe weight of said metal particles.

3. A process according to claim 1 wherein said metal has arecrystallization temperature of not greater than F.

4. A process according to claim 3 wherein said weldment inhibitorcomprises the reaction product of o-toluidene and formaldehyde andpolyethylene glycol.

5. A process according to claim 1 wherein said metal particles are zincparticles.

6. A process according to claim 1 wherein said mechanical flatteningforces are provided by tumbling said metal particles and aqueous mediain the presence of metal impactor comprising spherical metal particles.

References Cited UNITED STATES PATENTS JAMES E. POER, 'Primary ExaminerUS. Cl. X.R. 106-291 6/1960 McAdoW 106290

